Rosmarion Kerstin, Rothchild Robert
The City University of New York, John Jay College of Criminal Justice, Science Department, New York, New York 10019-1128, USA.
Appl Spectrosc. 2005 Mar;59(3):376-84. doi: 10.1366/0003702053585453.
3,6-Dibromophencyclone, 2, reacted with N-ethylmaleimide, 3a; N-n-propylmaleimide, 3b; and N-n-butylmaleimide, 3c; to form the corresponding Diels-Alder adducts, 4a, 4b, and 4c. The nuclear magnetic resonance (NMR) spectra of the adducts were studied at ambient temperatures at 300 MHz for proton and 75 MHz for carbon-13. Full proton assignments were achieved by high-resolution COSY45 spectra for the aryl proton regions. Rigorous assignments for protonated carbons were obtained with the heteronuclear chemical shift correlation spectra (HETCOR). Slow exchange limit (SEL) spectra were observed for both proton and carbon-13 NMR for each adduct, with slow rotation on the NMR timescales for the unsubstituted bridgehead phenyl groups. Endo Diels-Alder adduct stereochemistry was supported by substantial magnetic anisotropic shielding effects in the 1H NMR spectra of the alkyl groups. Proton NMR shifts are compared with those previously reported for the corresponding adducts, 5b and 5c, obtained from 3b and 3c, respectively, with the parent compound, phencyclone, 1. Results of ab initio molecular modeling calculations at the Hartree-Fock level using the LACVP* basis set for conformers of the dibrominated adducts, 4a-4c, are presented, together with HF/6-31G* results for the non-brominated adducts, 5a, 5b, and 5c. Novel aspects of this present work include: (a) attempts to quantitatively evaluate alkyl proton NMR shielding magnitudes in the adducts, relative to maleimide precursors, and (b) use of ab initio Hartree-Fock level calculations to try to reconcile adduct geometries with the observed shielding magnitudes. Our results here complement and extend studies of: (a) adducts of the parent phencyclone with straight-chain N-n-alkylmaleimides, and (b) adducts of 3,6-dibromophencyclone with other symmetrical dienophiles.
3,6 - 二溴苯环己酮（2）与N - 乙基马来酰亚胺（3a）、N - 正丙基马来酰亚胺（3b）和N - 正丁基马来酰亚胺（3c）反应，形成相应的狄尔斯 - 阿尔德加合物4a、4b和4c。在室温下，对加合物的核磁共振（NMR）谱进行了研究，质子谱在300 MHz下测定，碳 - 13谱在75 MHz下测定。通过芳基质子区域的高分辨率COSY45谱完成了全质子归属。通过异核化学位移相关谱（HETCOR）获得了质子化碳的严格归属。观察到每个加合物的质子和碳 - 13 NMR的慢交换极限（SEL）谱，未取代桥头苯基在NMR时间尺度上旋转缓慢。烷基的1H NMR谱中的显著磁各向异性屏蔽效应支持了内型狄尔斯 - 阿尔德加合物的立体化学。将质子NMR位移与先前分别从3b和3c与母体化合物苯环己酮（1）得到的相应加合物5b和5c的报道值进行了比较。给出了使用LACVP基组在哈特里 - 福克水平下对二溴化加合物4a - 4c构象进行的从头算分子模型计算结果，以及对非溴化加合物5a、5b和5c的HF/6 - 31G结果。本工作的新方面包括：（a）尝试相对于马来酰亚胺前体定量评估加合物中烷基质子的NMR屏蔽大小，以及（b）使用从头算哈特里 - 福克水平计算试图使加合物几何结构与观察到的屏蔽大小相协调。我们在此的结果补充并扩展了以下研究：（a）母体苯环己酮与直链N - 正烷基马来酰亚胺的加合物，以及（b）3,6 - 二溴苯环己酮与其他对称亲双烯体的加合物。
